This invention relates to wellheads for oil and gas wells. For convenience of description, reference will be made in this specification solely to oil wells, but it is to be understood that the invention extends equally to gas wells, gas or oil storage wells and geothermal wells.
In oil wells, there are two distinct phases, firstly exploration and drilling and secondly production. Once a successful well has been drilled, it must either be in production or kept sealed else the oil may escape.
Conventional technique is to secure a wellhead assembly to a surface casing on the ground (either on the surface or subsea), and to use that wellhead as a base both for drilling the well and for subsequent production. This has disadvantages. In the drilling phase, the wellhead is liable to damage as a result of the insertion and extraction of drill bits and drill rods, and from the evacuation through the well of the drilling mud and debris. Damage to the wellhead at this stage can necessitate expensive replacements, even before production has commenced.
On the other hand, production considerations demand a tightly engineered wellhead both to withstand the pressures generated from within the well and to ensure that the wellhead remains serviceable over a period of possibly as long as 40 years.
According to the invention, there is provided a method of drilling a well and preparing it for production, the method comprising the steps of providing, for the drilling stage, a drilling wellhead and when drilling is completed, sealing the well and production annulus; installing a production wellhead on the well and then unsealing the well bore and opening communication with the production annulus so that production can take place through the production wellhead whilst the production annulus is monitored.
With such a method, the drilling wellhead can be manufactured to relatively low cost specifications, and can be reused for the drilling of a number of wells. The drilling wellhead can even be hired out to well operators for drilling wells as it will only be in use at a particular well site for a limited period of time.
The production wellhead which is installed after the well has been drilled can thus be assured of being unlikely to suffer damage of the type possible when drilling rods and bits would have to pass through it were it to be in situ during the drilling phase.
After drilling has been completed through the drilling wellhead and prior to installation of the production wellhead, the well can be sealed by isolating the annular space surrounding the production casing, and by fitting a sealing cap within the production casing.
The drilling wellhead can then be removed from the casing hanger cluster to allow replacement by the production wellhead. A tubing hanger can be fitted into the production wellhead after unsealing of the casing bore and the annular space surrounding the production casing.
The production annulus can be provided with a seal ring which is axially movable between a sealing position for sealing the annulus and an open position for opening communication with the annulus. The seal ring can be mounted on a thread and can be moved between sealing and open positions by rotating the ring on its thread. This movement can be achieved with a tool inserted axially into the well.
Preferably the seal ring seals with the production wellhead and with the production casing when in the open position.
In a particularly preferred embodiment, the seal ring also forms a wellhead bowl protector.
The invention also provides wellhead apparatus comprising a casing hanger cluster comprising a plurality of concentric casing hangers, a seal ring which can be moved axially in the cluster to open and close communication between an annular space defined between the production casing hanger in the cluster and the next outermost casing hanger, a drilling wellhead and a production wellhead, the drilling and production wellheads being adapted to be fitted alternately on the cluster.
The seal ring may comprise a tubular body, with seals on its radially internal and radially external faces which seal with, respectively, the radially external face of one casing string and the radially internal face of a next outermost casing string.
Alternatively the seal ring may comprise a cylindrical body with at least two sets of seals on radially external faces, the seals being arranged at different diameters, the smaller diameter set of seals being arranged to seal with one casing hanger and the larger diameter set being arranged to seal with a next outermost casing hanger at a point where the next outermost casing hanger is not overlapped by the said one casing hanger.
The seal ring may have an axially extending skirt which acts as a bowl protector in the wellhead. The ring can have one set of seals at one end, and a second set carried by the lower end of the skirt.
The seal ring can have surfaces which make metal/metal seals with mating faces of the casing strings.
In one embodiment, the seal ring is in two parts which are held in a first relative position by a shear pin. A compression seal can be located between the two parts, the compression seal being uncompressed when the parts are in their first relative position, but being compressed when the parts move to a second relative position after the shear pin has sheared.
The seal ring can have an upper tapered surface for making a metal/metal seal with a corresponding surface of a production wellhead, and a lower tapered surface for making a metal/metal seal with a corresponding surface of a production casing hanger. The seal ring preferably also has O-ring seals for sealing with inner and outer casing strings and with a production wellhead.
The seal ring can be mounted on a thread, and can be moved between the seal open and seal closed positions by rotating the ring on its thread.
Further sealing/valving functions can be provided in the production wellhead, such as a ball valve (rotatable 90 degrees between open and closed positions) and/or a pressure relief plug.
The wellhead apparatus can also include a removable tubing hanger section, at the top of the tubing hanger, which is exposed to possible damage during use of the well, and which can be readily removed and replaced at low cost if it should become damaged.